1. Field Of The Invention
The present invention provides a system and method for non-intrusively detecting whether a network (which uses a collision detection access scheme) is open at one of its ends. In addition, the present invention also provides a system and method for detecting whether there is a short circuit (i.e., a shorted termination) in the network.
2. Related Art
Networks which link otherwise separate computers and other related devices (such as printers) together have become very popular, and in some cases are even a necessity to some companies. One type of popular network uses an access scheme known as collision detection to facilitate communication. The principle behind this scheme is that all of the separate computers or related devices (hereafter referred to as "devices") are connected to a network bus having at least two termination points. When one device is to communicate with another, it sends an "information packet" out onto the bus. This information packet contains such information as the destination device address, the sending device address and the data to be passed. In typical collision detection schemes, when an information packet is sent onto the bus (and propagates along the bus), a negative voltage is propagated.
In a collision detection access scheme, a device will send an information packet onto the network bus after first checking the network bus to make sure that no other information packets are being sent. After it detects that none are being sent, the device will also typically wait some specified period of time to make sure that an information packet is not about to be sent by another device. If the device does not believe that an information packet is about to be sent, it will send its information packet on to the bus.
Even with all the checking discussed above, however, it is still possible that more than one device will send an information packet substantially simultaneously. If this occurs, a "collision" is said to be the result. Since both information packets were sent out simultaneously, the negative voltage on the network bus will be greater (typically double) than it would have been if only a single device had sent one information packet. The result is that the data within the information packets involved in the collision are corrupted. Each device then retransmits its information packet at a slightly different time in order to avoid another collision.
On a network using a collision detection access scheme, the network bus must be properly terminated at all of its ends with a "terminator" of the same impedance as the conduction media of the network bus. Otherwise, the network will not work properly. In fact, the information packets (which are represented as a negative voltage having an AC component) will propagate in both directions on the data bus, and when this propagation comes to an improperly terminated end, it will propagate back the other way in a "constructive" manner. In other words, the voltage will typically increase to about double that of a normal information packet. Thus, it will appear as though a collision occurred.
The effect of an unterminated network can be seen from FIG. 2. Referring now to FIG. 2, at time (T) =0 a transmission occurs on the data bus. The left portion of the data bus is terminated by some impedance (as indicated by the greek letter Omega), whereas the right side is not. At T =0, a transmission (i.e. an information packet) is emitted at the transmission point shown. In this example, this transmission is shown to be minus one volt. At T =1, the -1 volts has propagated in both directions along the data bus. At T =2, the voltage has reached the end of the data bus having impedance, and no additional propagation occurs. However, on the right portion of the data bus where there is infinite impedance, the propagation reverses itself, and -2 volts begins to propagate in the other direction. By T =6, the entire data bus is at -2 volts. Thus, when this occurs, any information packet which is transmitted on the network will create -2 volts (or at least substantially more voltage than would be the normal case where the network was properly terminated) and thus all information packets will appear to have had a collision. Because of this, an improperly terminated network can cause serious problems.
Another problem which can occur on a network having a collision detection access scheme is a shorted termination. This means that the network bus has a short circuit. This typically results in an information packet appearing to have a positive peak voltage. In other words, the greatest flux of voltage created as a result of an information packet being sent by a device will be positive rather than negative. This can cause serious trouble in a network, since a large percentage of data errors can occur.
The magnitude of problems caused by either an improperly terminated endpoint of a network bus or else a short circuit can be appreciated from FIG. 1. Referring to FIG. 1, an example of a network set-up is shown, where devices labeled PC (for "personal computer") and WS (for "work station") are shown attached to a complex network. As can be seen, there are many endpoints involved with this network. If the impedance of these endpoints is not proper (i.e., an endpoint impedance terminator such as one at the ends of network segment 102 is removed) then if a device on the network transmits an information packet, it will appear to have had a collision, and will thus be discarded.
In typical networks such as the one contemplated by the example of FIG. 1, all devices of network segments which are connected via a repeater (such as network segments 102 and 112 connected by repeater 110) would be affected by an improper termination such as the one described above. Thus, for example, if one of the endpoints of the network segment 102 was improperly terminated, it would affect devices as far away as work station 136. The same is true if there were a short circuit somewhere in network segment 102. However, devices such as the bridge 140 would not propagate such a problem past that point. Nonetheless, such problems in a network environment can have serious and far-reaching repercussions.
Trouble shooting a network with such difficulties as discussed above requires many levels of tests before the actual problem is typically identified. Generally, a problem is identified using test devices such as Protocol Analyzers, Multimeters and Time Domain Reflectometers. When a problem in a network is suspected to be caused by an improper termination, tests are conventionally done which require the transmission of additional information packets onto the network. The same is true when the problem is suspected to be a shorted network. However, transmitting such additional information packets can potentially be detrimental to accurate testing of the network in that the time that it takes to generate these additional packets detracts from the observation functions of the test devices.
Thus, a need exists for non-invasive determination of improper termination and short circuits within networks.